The invention relates to a strain-gauge transducer comprising at least one strain gauge disposed between two pressure members which deform the strain gauge under the influence of an applied force, said strain gauge having a substrate provided with strain detectors and transfer means comprising segments of an elastic material between the substrate and the pressure members to transmit the applied force.
The invention also relates to force or weight measurement arrangement using such a transducer, and to a zone-access touch tablet or a touch-sensitive writing or signature tablet.
A strain-gauge transducer generally comprises a substrate carrying resistive elements whose resistance values vary with the deformations to which the substrate is subjected under the influence of the applied forces.
The characteristics of a transducer are defined by:
its sensitivity, which should be as high as possible, PA1 its linearity as function of the load, PA1 absence of fatigue, hysteresis, or breaking under high loads, PA1 its production costs, which should be low for large-scale uses.
In order to ensure that the weight or force measurement is linear and accurate the support which is used should have reproducible and linear mechanical properties. This is an advantage of gauges formed by a ceramic plate on which a resistive ink has been deposited by screen-printing. Indeed, the ceramic plate has a very large elasticity range, which is limited only by the breaking point of the ceramic plate itself. There is neither plastic deformation nor hysteresis.
On the other hand, a strain-gauge transducer is mainly intended for use in consumer applications, for example in scales or in force transducers of all kinds. The technologies used should therefore be free from the restraints associated with large series, in particular spreads of mechanical tolerances of the parts, which may lead to assembly problems and even to breakdown of the transducer.
The document FR 2,608,759 describes a weighing apparatus comprising a deformable plate carrying strain gauges disposed between pressure members formed by a base at the bottom and a panel at the top. The deformable plate is in contact with the pressure member by means of knife edges of an elastic material, for example of rubber. The knife edges are positioned by means of recesses in the pressure members and in the deformable plate. These recesses are formed by cutting or stamping, which implies a fairly low positioning accuracy. During assembly the rubber knife edges must be fitted into these recesses and must be positioned relative to the other one of the pressure members and the deformable plate.
This gives rise to mounting problems, particularly as a result of incorrectly machined substrates.
The inaccuracies as regards the positions of the knife edges cooperating with each element and as regards the distances between the plate and the pressure members concur, resulting in a force measurement with only a low precision.
For this reason, this type of weighing apparatus is mainly used for bathroom scales.